Some Carbonate Hydrocarbon Prospects in Albanides Fold-and-Thrust Belt
Prenjasi, Engjell 1; Dhima, Stavri 1;
Gjevori, Shkelqim 1; Arapi, Luan 1
(1)Polytechnic
University of Tirana, Tirane, Albania.
ABSTRACT
Exploratory works and integrated syntheses performed on the Albanides thrust belts have brought about discovery of several hydrocarbon fields and depiction of many prospects. But, many wells based solely on some surface flysch folds or on poor seismic events have resulted dry. Subsequently, new discoveries of oil and gas fields are becoming rare, but it does not mean poor perspective or that the time of “peak oil” has come, because there are many hydrocarbon prospects that are somewhat depicted. What’s more, it is still possible to detect even very shallow carbonate prospects and discover new “easy oil”. But, further hydrocarbon exploration needs desperately hi-tech and low-cost geological and seismic surveys, which can become real access to new hydrocarbon reserves. In other words, presently hydrocarbon exploration means harder work, because we are exploring for new reserves deeper and in more complicated geological settings.
Geological survey has taken the leading role of the all oil and gas exploratory methods in Albanides thrust belts over the past sixty years. So, based on geological data, realized was the discovery of the Gorishti and Delvina fields in the Cretaceous-Palaeogene carbonate reservoir. Meanwhile, seismic survey was the second phase, which resulted in some other discoveries, such as the Cakran and the Amonica oil fields in the same carbonate reservoir. On the other hand, the geological and seismic survey methods often have faced serious restrictions, which have brought about many dry wells. What’s happened? Why have even many foreign oil companies failed in Albania? The best response must come from review of the exploration philosophy and applied methodology. Thus, the negative results of oil and gas exploration in Albanides fold-and-thrust belts and along their homologues worldwide have come from improper techniques in the course of acquisition, integration, and evaluation of all the geological and geophysical data acquired. Subsequently, experimental works for picking up right acquisition and processing parameters, as well as providing obvious link of seismic events with known target horizons traced at surface by geological mapping and/or crossed by drilled wells remain a permanent must.
MAIN ACHIEVEMENTS AND FAILURES OF OIL AND GAS EXPLORATION IN ALBANIDES
Detailed geological surveys, as well as stratigraphic studies have been in the leading role of the all oil and gas exploratory methods carried out in the Outer Albanides thrust belts over the past sixty years. So, based on surface geology and few drilled wells data realized interesting discoveries, as the Gorishti oil field (1), Delvina gas condensate one (2), etc.
Caring out 2D seismic surveys was the second important step in Albania. So, based on some geological data backed by few seismic lines discovered several other fields as Finiq-Krane, Cakran, Amonice, etc. Nevertheless, since the projection of the wildcat, which stroke gas condensate in the very complicated geological setting of the Delvina field, there are some 30 years of dry exploration. What’s happened? Why even many foreign oil companies have not been successful in Albanides, Hellenides and other thrust belt regions worldwide? Most likely the negative results have come from implementation of impropriate exploration methodology in the following main aspects:
- Caring out insufficient surface geological mapping or former geological maps revisions, especially by foreign oil companies, which focus only on new seismic works.
- Improper orientation of seismic lines for recording hydrocarbon prospects’ structural features.
- Having not enough attention to cooperate with right selected hosting countries experts for looking into the all existing geological and geophysical data, especially by foreign oil companies, which has worked in Albania during the last twenty years.
- Doing wrong evaluation of the reliability of the all exploratory methods data, as well as their coherence during making up comprehensive integrated syntheses to detect possible oil and gas accumulations traps.
The four abovementioned issues should be solved successfully through tackling carefully the followings:
- Detailed revision of geological mapping as in the case of the Fushe-Verri prospect (fig.2, Prenjasi E. et al.,1992) and the all drilled wells data as in the case of the Delvina-4 formerly suspended well (fig. 5, 6). In both cases realized a detailed look into lithological, geochemical, structural and tectonic features of flysch deposits, which indicate whether a flysch fold may be reflection of an anticline structure at the limestone sequence or not? (Prenjasi E. et al., 1985, 1986, 1987, 1992).
- Timing between tectonic and secondary oil migration phases.
- Tracing field location of seismic lines, as well as selecting their acquisition and processing parameters that can provide obvious record of the undulations of the hydrocarbon target sequence everywhere, despite the geological setting complications. For example the Delvina 2D seismic profiles (fig.7) have provided reliable records, wherever working on outcrops of terrigenous deposits, whereas they have got nothing wherever shooting on the Mesozoic outcropped carbonate and evaporitic deposits the thrusted anticlines (fig. 5, 8).
- Reliability and coherence among the all oil exploratory methods data can bring about new discoveries even in the conditions of the lack of the seismic information. Thus, in the occasion of the synthesis of the project of the Del-9 wildcat realized a harmonic integration of the following non seismic clues:
- Location of Delvina prospect between the eroded anticlines of Krongji and Senica (fig. 4) and extension of their surface structural data towards that time Del-4 suspended well. So, based on the detection of microfaunistic boundary between the Middle and the Lower Oligocene flysch deposits at the depth of 2430m of the Del-4 well, as well as on the Lower Oligocene flysch thickness of some 1000m at the nearest drilled well of the Del-1, the expected top carbonate target depth at the Del-4 well interpreted at about 3500m (fig. 5, Prenjasi E. et al.,1980), i.e. some 3000m higher than the alternative of having only a continuous dip of 60 grade of the pericline closure of the Krongji eroded anticline towards the Del-4 well.
- b. Presence of the Spontaneous Polarization gradient in a value of some 5ml volt per 100m, at that time bottom interval of 2900-3050m of the Del-4 well (fig.6). This phenomenon backed coherently by the results of the geochemical analyses of the all samples picked up from the flysch interval of 2430-3050m of the Del-4 well (Prenjasi E. et al.,1980), because they detected presence of the element of Fe two-valiant in form of sulphide mineralization of pyrite. In other words the geochemical data detected the presence of a hydrocarbon flux, which is more emphasized in sandstone samples than in the clay ones and that increases towards the depth. Did this hydrocarbon flux come from an existing field or a remanence of destroyed one? Selection of the first alternative of this issue found out in the course of the paleotectonic development of the Delvina anticline during the Alpine orogenic phases which brought about the entrapment of the all hydrocarbon fluxes migrated after the Cretaceous–Eocene periods of time.
SPATIAL LOCATION OF CRETACEOUS- EOCENE LIMESTONE OIL PROSPECTS
In response of the negative results and their casual questions from numerous integrated exploratory field works and data syntheses have come out some invaluable practical experiences:
- Possibility of reflection of the Oligocene flysch folds at the hydrocarbon target of the Cretaceous-Paleogene limestone sequence depends on their spatial position in the course of tectonic development of the whole region that could bring about their birth and growing. Thus, in general, the anticline flysch folds, located in continuation of the eroded carbonate anticlines or known hydrocarbon fields, sometimes overlain transgressively by premolasse deposits of the Burdigalian to Serravallian, have a considerable reflection at the limestone sequence in question (Bakia H., et al., 1987, Prenjasi E., 1992).
- A complete depiction of the structural and tectonic relationships between a flysch fold and the nearest eroded carbonate anticlines is also very necessary. So, relatively big flysch anticlines, located in continuation of the eroded carbonate structures, can result in considerable reflection at the hydrocarbon target of the Cretaceous-Eocene limestone sequence. Also, small flysch folds expressed at the lower part of the Lower Oligocene flysch seal and located along direct continuation of the eroded anticlines must have substantial reflection at the limestone sequence. Whereas, small flysch folds encountered along the steep flanks of the big carbonate structures have not reflection at the limestone sequence in question (Prenjasi E., et. al., 1986, 1992). In other wards, if geological survey mapping makes out a closed flysch fold, the latter may or may not has expression at limestone sequence. But, if the flysch fold does not exist or it has relatively small size as in the case of the Borshi area, it may have only a slight terrace form reflection at the carbonate sequence (Fig. 10).
- Sometimes the carbonate oil traps may be conditioned by duplex pattern fault screened against the flysch deposits as it is shown in the eastern part of the Borshi profile (fig. 10). In the later occasion, as well as in the case of small narrow carbonate anticline of the Fushe-Verri prospect (fig. 2, 3); etc implementation of the Real resistivity section exploratory method has resulted more effective than the seismic one.
- Some drilled wells have proved that prevalence of the clay component in a flysch section brings about clay diapirism phenomenon and consequently the diminishing chances of reflection of the flysch fold at the limestone sequence (fig. 9-B, 10, 11, Prenjasi E., et. al., 1985,1986, 1989).
- Presumably, aggressive movement of the underground waters through high porosity carbonates is a potential threat of hindering hydrocarbon accumulation in small traps as in the cases of the Fushe-Verri and Fitore prospects (fig. 2, 3, Prenjasi E., et. al., 1986, 1992 ).
- The practical experiences considered above, etc, reveal that following the eroded anticline structures or known oil fields through detailed geological survey and hi tech seismic ones is the leading method of oil exploration in thrust belt regions.
CONCLUSIONS
Geological mapping must be in the leading role of the oil exploratory works across the thrust belt regions. So, following the eroded anticline structures or known oil fields through detailed geological survey and hi tech seismic ones have resulted as the most effective method of the oil exploration in thrust belt regions. Whereas focusing only on the seismic works, disregarding their orientation according to the geological setting and structural features of the oil prospects has brought about many dry wells.
Careful acquisition, processing and evaluation of the reliability of the all geological-geophysical data are a permanent must. So, a study on the possibility of reflection of flysch folds at the Cretaceous-Eocene limestone sequence must make out their lithologic, structural and tectonic features, as well as timing between folding and hydrocarbon migration phases. Also, seismic and all the other geophysical and geochemical exploratory methods must increase their solution capability through carrying out all necessary experimental methodical tests before shooting the all works volume.
Cooperation with properly selected hosting countries experts is another important key to realize the effective hydrocarbon exploratory works.
REFERENCES
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AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.